Power tools including battery operated tools are well-known. These tools typically include an electric motor having an output shaft that is coupled to a spindle for holding a tool. The tool may be a drill bit, sanding disc, a de-burring implement, or the like. Electrical power is supplied to the electric motor from a power source. The power source may be a battery source such as a Ni-Cad or other rechargeable battery that may be de-coupled from the tool to charge the battery and coupled to the tool to provide power.
The power source is coupled to the electric motor through a power switch. The switch includes input electrical contacts for coupling the switch to the power source. Within the switch housing, a moveable member, sometimes called a switch, is coupled to the input electrical contacts and to a wiper of a potentiometer. As the moveable member is pressed against the biasing component of the switch, it causes the input electrical contacts to close and provide current to one terminal of the electric motor and to the wiper of the potentiometer. The moveable member is biased so that the biasing force returns the moveable member to the position where the input electrical contacts are open when the moveable member is released. The current is coupled to a timing signal generator, such as a 555 circuit, through the potentiometer. As the member or trigger continues to be pulled against the biasing force so that the wiper reduces the resistance of the potentiometer from an open circuit to a low resistance or short circuit condition, the level of the current supplied to the timing signal generator increases.
The output of the timing signal generator is coupled to the gate of a solid state device, such as a MOSFET. The source and drain of the solid state device are coupled between a second terminal of the electric motor and electrical ground. In response to the timing signal turning the solid state device on and off, the motor is selectively coupled to electrical ground through the solid state device. Thus, as the timing signal enables the solid state device to couple the motor to electrical ground for longer and longer intervals, the current flows through the motor for longer intervals. The longer the motor is coupled to power, the faster the electric motor rotates the output shaft of the motor. Consequently, the tool operator is able to vary the speed of the motor and, correspondingly, the rotational speed of the tool in the spindle by manipulating the trigger for the power switch.
The timing signal generated by the timing circuit selectively couples the motor to the power source because it alternates between a logically on-state and a logically off-state. During the logically off-state, the motor is no longer coupled to the power source. The windings in the motor, however, still have current in them. To provide a path for this current, a freewheeling diode is provided across the terminals of the motor.
The trigger of the power switch is also coupled to two sets of contacts. One of these contact sets is called the bypass contact set. When the trigger reaches the stop position of its travel against the biasing component, it causes the bypass contacts to close. The closing of the bypass contacts causes the current through the motor to bypass the solid state device and be shunted to electrical ground. This action enables the motor to remain continuously coupled to the power source and reach its maximum speed.
The other set of electrical contacts controlled by the switch trigger are the brake contacts. These contacts are closed when the trigger is at the fully biased off position. As the trigger is moved against the biasing force, the brake contacts open. The brake contacts couple one terminal of the electric motor to the other terminal of the motor. In response to the trigger being released from a position that enables power to be supplied to the motor, the brake contacts close to provide a current path through the motor for dynamic braking of the motor. This enables the motor to stop more quickly than if the motor simply coasted to a stop under the effects of friction.
While the power switch described above is effective for tool speed control, it suffers from some limitations over the life of the battery operated power tool. One drawback arises from the mechanical interaction of the various components. Specifically, the moveable member, biasing component, wiper arm, and electrical contact sets, all move with respect to one another. This movement causes friction and the moveable components become worn over time. This wear occurs even when the trigger and the components coupled to the trigger are manipulated and no battery source is coupled to the tool. Additionally, dropping the tool or other rough treatment of the tool may result in disruptions of the mechanical linkages between the components. These disruptions may adversely impact the mechanical synchronization of the switch components required for supplying power to the electric motor. For example, if the linkages between the moveable member and the various electrical contacts are disrupted, the battery current may be coupled through the input contacts before the brake contacts open. This action would cause a short circuit to electrical ground and may cause irreparable harm to the switch.
Another limitation of known power switches is the effect of carrying the battery current through the switch. When the battery current is first applied to the contacts, the current level may be sufficient to cause arcing. Arcing may cause the contacts to become pitted or otherwise damaged. Additionally, large currents also tend to heat the components within the switch. Consequently, the switch may require a heat sink or a larger volume to dissipate heat within the switch. The larger size of the housing for the switch may also impact the design of the tool housing to accommodate the switch geometry. Another factor affecting the geometry or size of the switch housing is the potentiometer that generates the variable speed signal. Typically, the distance traveled by the wiper of the potentiometer is approximately the same as the distance traveled by the trigger. In many cases, this distance is approximately 7 mm and this distance must be accommodated by the potentiometer and the housing in which the potentiometer is mounted.
The direction of motor rotation depends upon whether the battery current flows through the motor from the first terminal to the second terminal or vice versa. Because bidirectional rotation of battery operated tools is desirable, most tools are provided with a two position switch that determines the direction of battery current through the electric motor. In some previously known switches for battery operated tools, this two position switch is incorporated in its own housing that is mounted to the switch housing. The additional two position switch housing may exacerbate the space issues already noted. In other known switches, the two position switch may be integrated within the switch housing. This arrangement, while perhaps smaller than the two housing construction, adds another set of contacts to the switch with the attendant heat or contact deterioration concerns that arise from the motor current flowing through these contacts.
Another limitation of known power switches relates to the torque control for power tools. In some battery operated tools, mechanical clutches are used to set a torque limit for the tool. When the resistance to the rotation of the tool causes the torque generated by the tool to increase to the torque limit, the clutch slips to reduce the torque. The torque may then build again until it reaches the limit and the clutch slips again. The iterating action of clutch slippage followed by renewed torque buildup is sensed by the operator as vibration. This vibration informs the operator that the tool is operating at the set torque limit. This slippage also causes wear of the mechanical components from friction and impact.